Cochlear implants (inner ear prostheses) are an option for helping profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids, which just apply an amplified and modified sound signal, a cochlear implant is based on direct electrical stimulation of the acoustic nerve. The intention of a cochlear implant is to stimulate nervous structures in the inner ear electrically in such a way that hearing impressions most similar to normal hearing are obtained.
FIG. 1 shows a conventional cochlear prosthesis. The cochlear prosthesis essentially consists of two parts, the speech processor 101 that is typically positioned externally proximate the ear, and the implanted stimulator 105. The speech processor 101 typcially includes the power supply (batteries) of the overall system and is used to perform signal processing of the acoustic signal to extract the stimulation parameters. The stimulator 105 generates the stimulation patterns and conducts them to the nervous tissue by means of an electrode array 107 that extends into the scala tympani 109 in the inner ear. The connection between the speech processor 101 and the stimulator 105 is established either by means of a radio frequency link (transcutaneous) using primary coils' 103 and secondary coils within stimulator 105, or by means of a plug in the skin (percutaneous).
One basic problem in cochlear implant applications is spatial channel interaction. Spatial channel interaction means that there is considerable geometric overlapping of electrical fields at the location of the excitable nervous tissue, if different stimulation electrodes (positioned in the scala tympani) are activated. Thus the same neurons are activated if different electrodes are stimulated. Spatial channel interaction is primarily due to the conductive fluids and tissues surrounding the stimulation electrode array.
At present, the most successful stimulation strategy is the so called “continuous-interleaved-sampling strategy” (CIS) introduced by Wilson B S, Finley C C, Lawson D T, Wolford R D, Eddington D K, Rabinowitz W M, “Better Speech Recognition with Cochlear Implants,” Nature, vol. 352, 236-238, July 1991, which is incorporated herein by reference.
Signal processing for CIS in the speech processor typically involves the following steps:                (1) splitting up of the audio frequency range into spectral bands by means of a filter bank;        (2) envelope detection of each filter output signal; and        (3) instantaneous nonlinear compression of the envelope signal (map law).        
According to the tonotopic organization of the cochlea, each stimulation electrode in the scala tympani is associated with a band pass filter of the external filter bank. For stimulation, symmetrical biphasic current pulses are applied. The amplitudes of the stimulation pulses are directly obtained from the compressed envelope signals (step (3) above). These signals are sampled sequentially, and the stimulation pulses are applied in a strictly non-overlapping sequence. Thus, the problem of spatial channel interaction is defused and a comparatively precise definition of electrical fields in the cochlea is achieved.
For example, consider a 12-channel CIS-system with a maximum overall stimulation rate of 18 kpps. Assuming that each channel is addressed once in a cycle, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal.
The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be chosen arbitrarily short, because the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For an overall stimulation rate of 18 kpps, the phase duration is 27 μs, which approaches the lower limit.
A stimulation strategy closely related to CIS is the so-called “N-of-M” strategy, wherein only the N electrode channels with maximum energy are selected out of the total number of M channels during each stimulation cycle, as described by Wilson B S, Finley C C, Farmer J C, Lawson D T, Weber B A, Wolford R D, Kenan P D, White M W, Merzenich M M, Schindler R A, “Comparative studies of speech processing strategies for cochlear implants,” Laryngoscope 1998; 98:1069-1077, which is incorporated herein by reference. Typically, number M is constant and equal to the overall number of usable channels. Thereby the instantaneous stimulation rate of a selected channel is increased by a factor of M/N. Interestingly, N-of-M strategies do not seem not to improve speech perception as compared to standard CIS, as described in Ziese M, Stützel A, von Specht H, Begali K, Freigang B, Sroka S, Nopp P, “Speech understanding with CIS and N-of-M Strategy in the MED-EL COMBI 40+ system,” ORL 2000; 62:321-329, which is incorporated herein by reference.
One disadvantage of N-of-M strategies (with constant M) is that neurons or ensembles of neurons may suffer “micro-shocks”, if electrode channels are switched from “inactive” to “active”. For example, consider a situation where a train of supra-threshold pulses is switched on at a particular electrode. The initial pulse in the train will cause action potentials in the majority of neurons that are close to the electrode, followed by a refractory period in which a more limited neural response can be elicited. The majority of the neurons will continue to be at similar refractory states, until sufficient time has passed to cause a sufficient distribution of refractory states. Thus, for at least an initial period of time, the majority of neurons will respond in the same manner to each pulse due to their similar refractory state, as described by Wilson B S, Finley C C, Farmer J C, Lawson D T, Zerbi M, “Temporal representation with cochlear implants,” Am. J. Otology, Vol. 18, No. 6(Suppl), S30-S34, 1997, which is incorporated herein by reference.
In standard CIS, periods with no activity at particular electrodes do not occur, since each electrode is stimulated in each cycle, and minimum pulse amplitudes are usually close to or slightly above thresholds. So even when there is no spectral energy present in a particular frequency band, the associated electrode will be active, keeping neurons in different refractory states. Additionally, a number of neurons may be kept busy because of activity of neighboring channels. In this respect, spatial channel interaction can have an (unintentional) advantageous effect.